The revolutionary capabilities of next-generation autonomous underwater vehicles

Offshore survey work has long been economically and environmentally demanding. In search of a modern solution for its own oceanographic surveys, the National Oceanography Centre (NOC) has developed a revolutionary autonomous underwater vehicle (AUV) range known as ‘NOC Autosub’ vehicles. These AUVs have the capability to operate in some of the deepest and most remote parts of the ocean. One particular variant, the NOC Autosub Long Range vehicle, can operate without the need for specialist launch and recovery equipment and, for less remote locations, without the need for a support vessel at all. This transformative technology is opening up new possibilities for offshore work that is traditionally completed by crewed vessels.

The first NOC Autosub vehicle missions took place in 1997. Since then, NOC Autosub AUVs have been used extensively for ocean research in remote and hostile environments where endurance, reliability and the ability to operate independently with little or no vessel support are essential requirements. To accommodate continuously changing research demands, NOC Autosubs have been designed to provide excellent power and flexibility for different chemical sensor payloads and imaging equipment.

The NOC Autosub Long Range is an electrically powered underwater vehicle that operates at depths of up to 1,500m or 6,000m (dependent on vehicle variant) and can carry a sensor package with different options such as sidescan multibeam, sub-bottom profiler or a camera system. The AUV can also be equipped with a flexible package of additional chemical sensors separate to its own navigation sensors and equipment. The vehicle itself is engineered to have a dry weight of less than one ton and maximum length of 4.2m, ensuring that it is easily handled with a single point lift.

Since navigational accuracy of the vehicle is crucial in many applications, NOC Autosub 5 features an underwater navigation system which, when in survey mode and achieving a bottom lock, is accurate to within 0.05% error for distance travelled. When the vehicle surfaces, any deviation is corrected via GPS. Where greater navigational accuracy is required, an acoustic modem on the vehicle works with ultra-short baseline (USBL) acoustic technology, offering the highest performance in positioning accuracy for applications such as surveying pipelines or cables.

How proven and mature is the technology?

The NOC Autosub has been used extensively for scientific research and understanding the ocean around the globe for several years, including successful operations under ice. A recent mission comprised of surveying a decommissioned oil installation in the North Sea demonstrated the success of using NOC Autosub’s high-tech, low-impact monitoring capabilities to detect environmental impacts at the sites. During these surveys, the vehicle was shore launched and recovered, undertaking a 400km total commute and marking a major milestone in the ambition of supporting the industry transition towards net zero targets.

NOC Autosub has also completed survey work within marine protected areas, capturing over 40,000 seafloor images using a high-specification camera system. The vehicle was able to generate georeferenced, colour-corrected conventional still images, as well as texture maps and corresponding microtopographic maps of the seafloor. This was a triumphant demonstration of the vehicle’s ability to operate very close to the seabed, providing more superior seafloor mapping data than comparable systems usually deployed by wire from a survey ship. These missions clearly demonstrated the commercial viability of the vehicle and made a clear case for NOC to invest in building additional vehicles for use in scientific research.

A solution for today’s challenges

Vehicles that have the endurance of NOC Autosub lend themselves to a wide range of applications such as deep-water geophysical survey, construction support, marine archaeology, oceanography and search and recovery.

For many, the major benefit of this technology is the fact that it can often be shore launched and recovered without the need of a vessel, which means it can provide both a more cost-effective and environmentally friendly solution than the traditional alternative of a crewed vessel with remotely operated underwater or survey equipment. Furthermore, with its ability to stay close to the seafloor, NOC Autosub is well insulated from adverse weather conditions that might disrupt alternative survey solutions.

While the NOC Autosub has completed continuous operations over long periods, mission duration is ultimately dependent on the power requirements of the final sensor package, with two to three weeks being the typical range for normal survey applications. The vehicle will typically surface daily to send data and is also able to receive any new piloting instructions when surfaced, if so required.

Challenges to the adoption of AUV technology

While AUVs may not form a viable solution for every application, many of the existing challenges of the technology are perceptual. First, there can be concerns about the lack of near-real-time data processing. When survey data is traditionally collected onboard a vessel, imagery can be processed and reviewed on the fly and small adjustments made to ensure both the quality and coverage of the data are sufficient. It is true that with a vehicle such as NOC Autosub Long Range, much of its sensor data is only downloadable to process when it is recovered at the end of its mission. In practice, however, the vehicle’s ability to fly accurately on course and close to the seabed tends to ensure extremely high-quality results.

Second, there is the perception that both surface and underwater autonomous vehicles could be more impacted by adverse weather than traditional methods. When it comes to NOC Autosub Long Range, this is less of a concern as the vehicle can be operated in low power mode or utilize its own recently developed anchoring system to wait out extreme weather if required.

Finally, survey operators and customers are reluctant to move from traditional and established solutions using conventional vessels to trialling new autonomous equipment. Changing the mindset is a difficult obstacle to overcome, but with any technology there will be early adopters and those who choose to adopt later. Running parallel to this however is the unrelenting demand for more environmentally friendly solutions. This is becoming more and more prevalent as survey work supporting green initiatives continues to grow and these clients are likely to favour greener survey solutions.

The wider autonomous underwater market

As a wide range of AUVs are available with different sizes and pricing points, it is important to have some broad understanding of how to consider and categorize different offers. Autonomous underwater vehicles can broadly be broken down into categories based on their size, payload, depth rating and endurance. Any AUV design will be a compromise of these factors. Ignoring for now the very large underwater autonomous vehicles used predominantly for defence applications, at the smallest scale micro AUVs and then person-portable AUVs are both designed for short-range missions with low current drain sensors. Next in size are long-range AUVs such as NOC Autosub Long Range, and finally there are work-class AUVs – large, high-power vehicles such as the NOC Autosub 5. Work-class AUVs are capable of powering multiple high-resolution sensors that demand considerable power.

The NOC Autosub 5, a ‘big brother’ to the Autosub Long Range, was developed over the same period and is well proven for operations requiring higher power sensors and faster operations (cruise speed typically 1.4m/s). Its primary objective is to be launched from a ship to image the seabed in high resolution by operating multibeams used for topographic mapping, sidescan sonars used for acoustic imaging of the seabed, sub-bottom profilers used to see what is in the first few metres under the seabed, and camera systems. The six-metre, two-ton vehicle is able to explore depths up to 6,000m with a 300km range and 72-hour endurance. It is launched and recovered using NOC’s winch davit launch and recovery system, which allows operation in the open ocean, up to sea state 4.

This vast difference in size, payload, depth rating and endurance is important to recognize when considering which AUV best suits specific survey applications.

Conclusion

AUV technology now offers new opportunities to conduct surveys in all types of applications, including bathymetric, hydrographic, archaeological and ecological surveys. The vehicles provide a safe and reliable route to inspect and assess engineering structures, construction sites, pipelines and cables. Since there are many autonomous vehicle types and options available in a ‘noisy’ market, careful research is required for operators to pick vehicles that best suit their requirements. There are likely to be different adoption rates to using both underwater and autonomous surface technology compared with more conventional survey approaches, and this may prove to be an important differentiator and competitive advantage both from carbon reduction and operational capability perspectives.

Ultimately, these revolutionary vehicles present a golden opportunity to modernize offshore survey work and to increase the quality and quantity of data being gathered from the ocean.